1. Field of the Invention
The present invention is directed to x-ray diagnostics installations having an x-ray converter with a luminescent image storage screen, and in particular to a read system for reading the image from the storage screen and generating a visual image therefrom.
2. Description of the Prior Art
An x-ray diagnostics installation is disclosed in U.S. Pat. No. 4,485,302 having an x-ray converter with a luminescent sceen which latently stores the x-radiation image, and which has a read-out system for the screen which causes the storage screen to luminesce by emitted rays of an additional read-out radiation source. The read-out system includes a detector for acquiring light emitted by the luminescence screen, and which converts the optical signals into electrical signals, which are then reproduced on a monitor. As disclosed in the patent, the luminescence screen consists of a storage luminophore functioning as a radiation-sensitive transducer when irradiated with x-rays, causing the generation of electrons which are stored in potential wells or traps having a higher surrounding energy level. During read-out, the storage luminescence screen is scanned pixel-by-pixel by an additional radiation source, for example, by a laser, so that the electrons stored in the traps are excited and can fall back into lower energy levels, the energy difference being emitted as a light quantum.
During scan of a line by the laser beam, the light emitted from the storage screen is acquired by a light conductor optics and is projected onto the light-sensitive entry face of a detector. The output signal of the detector is supplied to a conventional video chain for reproduction of the x-ray image.
In conventional systems, a photo-multiplier is used as the detector, the photo-multiplier converting the individual light quanta into a quasi-continuous current signal. The detector system thus has inherent inertia such that a chronological resolution of the individual events is not possible, nor is this type of resolution desired. The continuous analog signal generated as described above is supplied to an analog-to-digital converter in a standard manner, the analog-to-digital converter converting the magnitude of the generated voltage or of the current into a digital number. After further processing, the digital signal is again converted into an analog signal in a digital-to-analog converter. This analog signal is capable of being reproduced on a monitor. Thus a continuous signal is generated from a discrete pulse sequence, this continuous signal being in turn converted into discrete numerical values. Errors arise as a consequence of the analog processing, particularly due to the characteristic of the detector and of the amplifier. Errors also arise due to the necessity for multiple conversions. Such errors must be eliminated, insofar as possible, by suitable calibration.